1. Field of the Invention
This invention relates to aqueous slurries and particularly relates to nonsettling and flowable aqueous slurries of sodium dithionite that remain in pumpable form without significant expansion, settling, or gellation. It particularly relates to the manufacture of aqueous dithionite slurries for woodpulp bleaching that have dependable and reproducible stability.
2. Review of the Prior Art
Sodium dithionite, commonly termed sodium hydrosulfite and, less correctly, sodium hyposulfite, is a powerful reducing agent that has long been used for bleaching, particularly for bleaching textiles and wood pulps such as groundwood and semi-chemical pulps.
When anhydrous sodium dithionite crystals are dissolved under either aerobic or anaerobic conditions to make a large quantity of aqueous solution, the resulting solution cannot be stored for use over a long period of time. Due to hydrolytic decomposition at the natural pH of the sodium dithionite solution, decomposition will proceed rapidly from that point by self-propagation because the decomposition products create an acidic condition which accelerates the decomposition.
Aqueous solutions of the dithionite will decompose at a commercially tolerable rate, however, if stabilized by additives such as are disclosed in U.S. Pat. Nos. 3,819,807 and 3,985,674 These additives include chelating agents, sodium carbonate, sodium tripolyphosphate, sodium hydroxide, and amines.
Although such stabilized solutions can be protected from decomposition for long enough periods for shipment and routine commercial use under suitable conditions, it has been more common practice to store the anhydrous dithionite crystals under a dry, inert gas in a sealed container. Even though the crystals are thereby acceptably stable chemically for long periods, they begin to decompose much faster as soon as exposed to the air and moisture when the container is opened for use thereof.
Furthermore, commercially available solutions of sodium dithionite are expensive to transport because they are typically at concentrations of 12-13.5%, when combined with suitable additives, and, additionally, generally require refrigeration for shipment and storage. Thus, the transport of about seven times as much water as product tends to cause the sale of this commodity to become distance-dependent. In consequence, slurries have seemed to offer an inviting means to avoid or at least to minimize the cost of storage and difficulties associated with solution forms of sodium dithionite, without decreasing the convenience that the purchaser derives from solutions.
However, the economical preparation, stabilization, handling, and shipping of such slurries is not simple. Adequate suspension without agitation, so that pumping can be done from a tank truck after shipment, is also not easy. In fact, after considering the variety of processes that are available for manufacturing sodium dithionite, including the indigenous by-products, crystal structures, and the like, the complexities of the concept are readily appreciated. Moreover, slurries have not been as widely investigated nor as commercially utilized as other forms of sodium dithionite.
U.S. Pat. No. 3,536,445 describes a process for making sodium dithionite from sodium-zinc alloy by initially producing zinc dithionite and then converting it to sodium dithionite by adding caustic soda. After removal of the zinc hydroxide by filtration, the dihydrate of sodium dithionite is "salted out" of the mother liquor with sodium chloride and alcohol to form a slurry.
U.S. Pat. No. 3,804,944 gives some stability storage data for 30% slurries (18.5% formate-derived and 11.5% zinc-derived sodium dithionite) containing 1-8% caustic soda (dithionite basis). Tests showed that these slurries required frequent agitation to prevent caking and handling difficulties.
U.S. Pat. No. 3,839,217 shows that by reducing the particle size of the sodium dithionite crystals and/or introducing a suspending or thickening agent into a liquid containing the crystals, such as alcoholic brine, it is possible to form a fluent, homogeneous, pourable dispersion of the solid dithionite particles which is chemically and physically stable for long periods of time, provided that a material, such as the salt in the brine and/or an alcohol, be present which suppresses the dissolution of the dithionite so that the dispersion can be stored at about 20.degree. C. The majority of the particles should be about 0.6-0.8 micron in size. Methylcellulose, hydroxyethyl cellulose, polyvinyl alcohol, guar gum, and other common thickening, dispersing, or suspending agents can be used. The thickened dispersion exemplarily has a Brookfield viscosity of 9,000 cps and contains up to 34% Na.sub.2 S.sub.2 O.sub.4.
U.S. Pat. No. 3,839,218 provides a method for maintaining a dispersion of crystalline zinc or alkali metal dithionite hydrate by continuous or periodic mechanical agitation so that the crystals can be stored for long periods without decomposition, the dispersing medium being aqueous or nonaqueous and containing a material which suppresses dissolution of the dithionite solids. The pH of the liquid must be at least 6.5, the viscosity of the dispersion must be below about 50,000 centipoises, and the suppressing material may be a water-soluble organic compound or a saturated brine or mixtures thereof. A thickening and suspending agent can be used. Suitable agents include polysaccharides, water-soluble polymers, and proteins of moderate molecular weight. Exemplary agents include guar gum, gum tragacanth, gelatin, and starch.
U.S. Pat. No. 4,283,303 (the disclosure of which is incorporated by reference) discloses a method for making substantially stable slurries containing 30-35% by weight of sodium dithionite by evaporating sodium dithionite solutions while maintaining the heating medium at 220.degree.-250.degree. F. and the solution and slurry at 110.degree.-155.degree. F. under a vacuum of at least 25 inches Hg and by promptly cooling the resultant slurry while agitating it. The vacuum is preferably 26.5-27.5 inches Hg. Zinc-derived sodiation liquor is the preferred sodium dithionite solution to which 4-5% by weight of the sodium dithionite, NaOH, and a chelator, as a stabilizing agent, are added.
Although these evaporated slurries have excellent stabilization qualities, they have developed problems with settling which has occurred over a period of 2-5 days and especially under the vibrations produced by tank car shipment. Such settling, and subsequent hardening, has resulted in shipments which could not be unloaded by pumping as would normally be done.
Slurries are utilized as foods, coatings, paints, dyes, explosives, oilwell fluids, and the like, and they often include natural or synthetic gums to form a liquid colloidal system in which the solid particles are dispersed. The gums typically impart viscosity to sols in which they are incorporated and thereby function as thickeners. Such a gum-containing fluid system, without the solid particles, is identified as a sol and is more accurately termed a hydrosol when based on water.
Numerous natural and synthetic gums are widely used for manufacturing hydrosols. Favored gums for many hydrosols are galactomannan gums such as guar gum, which is derived from the endosperm of the guar plant, Cyamopsis tetragonolobus. Other water-soluble gums which are increasingly utilized are the xanthonomas hydrophilic colloids, commonly termed xanthan gums, which may be produced by the action of various bacterial species of the genus Xanthonomas on carbohydrates (and like materials). The fermentation product of the reaction of the bacteria Xanthonomas campestris, a preferred species, on carbohydrates is commercially available as "Kelzan XC Polymer", made by Kelco Corporation of San Diego, Calif.
Xanthan gum is an excellent and widely used suspending and viscosity building agent. Some of its particular uses are in oil well fluids, paint, sprays, and cleaning fluids. Xanthan gum, however, has a few disadvantages. It is very difficult to disperse and wet in water or brine so that hydration can take place. A high degree of shear is usually necessary in order to wet each gum particle. Once dispersal and wetting are accomplished, the hydration of the gum, as evidenced by the development of viscosity, is quite rapid. Xanthan gum and guar exhibit very different rheological characteristics, have different molecular configurations, and are obtained from entirely different sources. Various proprietary xanthan gums, having slightly different molecular structures and rheological properties by use of mutant strains of X. Campestris, are available from several manufacturers.
A need existed for a stable dithionite hydrosol composition having such pseudoplastic properties that it would be readily storable, even though subject to vibrations during tank car or tank truck shipment to a textile mill or to a pulp mill, for example, and readily pumpable when thereafter delivered to a storage tank for dilution to a solids content of 12-13% and short-term storage until needed, such as for bleaching textiles or woodpulp. However, attempts to use both guar gum and xanthan gum as suspending agents for sodium dithionite crystals had demonstrated that they had surprisingly unpredictable tendencies to form either gels or settled slurries, even during quiescent storage, so that there was a need for a process that could produce physically stable slurries.
U.S. Pat. No. 4,534,954 (the disclosure of which is incorporated by reference) is directed to the preparation of aqueous sodum dithionite slurries which are non-settling during shipment thereof, and are thereafter pumpable. As textile bleaching compositions, the slurries comprise, on a weight basis, at least about 36% of commercial sodium dithionite, at least about 3% of sodium hydroxide, at least about 0.25% of a chelate, and at least about 0.25% of a xanthan gum. It also discloses woodpulp bleaching compositions which additionally contain sodium tripolyphosphate.
When the process of U.S. Pat. No. 4,534,954 was utilized to produce slurries for woodpulp bleaching, gelling of the slurries occurred frequently and unpredictably. There was consequently a clear need for a process, utilizing the common strain of X. Campestris (e.g., the Kelzan grade), that could furnish dependable and consistently reproducible slurries capable of retaining their pumpable characteristics, with no tendency toward significant expansion, settling, or gellation, under usual commercial shipping and storing conditions.